Nickel-based alloy for hot forging die, hot forging die using same, and method for manufacturing forged product

ABSTRACT

Provided is a Ni-based alloy for hot forging die having high compressive strength at high temperature and good oxidation resistance that is capable of suppressing work environment deterioration and shape deterioration. A Ni-based alloy for hot forging die in the present invention includes, by mass, 7.0 to 12.0% W, 4.0 to 11.0% Mo, 5.0 to 7.5% Al, and 0.5 to 7.5% Cr, and the balance of Ni with inevitable impurities. In addition, the Ni-based alloy for hot forging die may further include 0.5 to 7.0% by mass Ta and may further include one or more elements selected from, by mass, 0.001 to 0.5% Zr, 0.001 to 0.5% Hf, 0.001 to 0.2% a rare-earth element, 0.001 to 0.2% Y, and 0.001 to 0.03% Mg. The Ni-based alloy for hot forging die may have a 0.2% compressive proof strength of at least 500 MPa at a test temperature of 1000° C. and a strain rate of 10−3/sec.

TECHNICAL FIELD

The present invention relates to a Ni-based alloy for hot forging die, a hot forging die using the same, and a method for manufacturing a forged product.

BACKGROUND ART

In forging of a product made of a heat-resistant alloy, the forging workpiece is heated to a predetermined temperature to lower its deformation resistance. Heat-resistant alloys have high strength even at high temperature. Thus, hot forging dies to be used for forging of those alloys are required to have high mechanical strength at high temperature. Also, in hot forging, if the temperature of the hot forging die is lower than the temperature of the forging workpiece, the workability of the forging workpiece will decrease due to die chilling. For this reason, forging of a product made of a poor workability material, such as Alloy 718 or a Ti alloy, is forged by heating the hot forging die as well as the workpiece. Then, the hot forging die must have high mechanical strength at a temperature equal to or close to the temperature to which the forging workpiece is heated. Ni-based superalloys usable in hot forging in which the die temperature in air is 1000° C. or higher have been proposed as hot forging dies satisfying this requirement (see Patent Documents 1 to 3, for example).

Note that the hot forging in the present invention includes hot die forging in which the temperature of the hot forging die is raised to near the temperature of the forging workpiece, and isothermal forging in which the temperature of the hot forging die is raised to the same temperature as the forging workpiece.

REFERENCE DOCUMENT LIST Patent Documents

-   Patent Document 1: JP S62-50429 A -   Patent Document 2: JP S60-221542 A -   Patent Document 3: U.S. Pat. No. 4,740,354 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The above Ni-based superalloys are advantageous in that the compressive strength is high at high temperatures, but are not so in terms of oxidation resistance since fine nickel oxide scale scatters from the die surface during cooling after the heating in air and may possibly deteriorate the work environment and the die shape. The problems of oxidation of the die surface and scattering of scale are major problems in maximizing the advantage of the usability in air.

An object of the present invention is to provide a Ni-based alloy for hot forging die having high compressive strength at high temperature and good oxidation resistance and being capable of suppressing work environment deterioration and shape deterioration, a hot forging die using the same, and a method for manufacturing a forged product using the hot forging die.

Means for Solving the Problems

By studying the problems of work environment deterioration and shape deterioration due to oxidation of the die surface and scattering of scale resulting from it, the present inventor found a composition having high compressive strength at high temperature and good oxidation resistance and arrived at the present invention.

Specifically, one aspect of the present invention is a Ni-based alloy for hot forging die including 7.0 to 12.0% W, 4.0 to 11.0% Mo, 5.0 to 7.5% Al, and 0.5 to 7.5% Cr and the balance of Ni with inevitable impurities.

In the present invention, in addition to the above-mentioned composition, 0.5 to 7.0% of Ta may further be included.

In the present invention, in addition to the above-mentioned composition, one or more elements selected from 0.001 to 0.5% of Zr, 0.001 to 0.5% of Hf, 0.001 to 0.2% of a rare-earth element, 0.001 to 0.2% of Y, and 0.001 to 0.03% of Mg may further be included.

Furthermore, in the present invention, the Ni-based alloy for hot forging die preferably has a 0.2% compressive strength of at least 500 MPa at a test temperature of 1000° C. and a strain rate of 10⁻³/sec.

The Ni-based alloy for hot forging die more preferably has a 0.2% compressive strength of at least 350 MPa at a test temperature of 1100° C. and a strain rate of 10⁻³/sec.

Another aspect of the present invention is a hot forging die using the above-mentioned Ni-based alloy for hot forging die.

The hot forging die using the above-mentioned hot die Ni-based alloy more preferably includes an antioxidant coating layer on at least one of a shaping surface and a side surface of the hot forging die.

Further another aspect of the present invention is a method for manufacturing a forged product, including: a first step of heating a forging workpiece; and a second step of hot-forging the forging workpiece heated in the first step by using the above-mentioned hot forging die.

In the forged product manufacturing method, the second step is more preferably performed with the hot forging die heated to at least 1000° C.

Effects of the Invention

By using the Ni-based alloy for hot forging die according to the present invention, which has high compressive strength at high temperature and good oxidation resistance, it is possible to suppress work environment deterioration and shape deterioration in hot forging.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the oxidation resistance in present invention examples and a comparative example under the test conditions simulating heating and cooling in repetitive use of a die.

FIG. 2 is a graph showing the compressive proof strength at high temperature in working examples and a comparative example.

FIG. 3 is a photograph showing the effect of preventing oxidation of a die surface by application of an antioxidant.

MODES FOR CARRYING OUT THE INVENTION

A Ni-based alloy for hot forging die of the present invention will now be described in detail. Units in a chemical composition are given in mass %.

<W: 7.0 to 12.0%>

W dissolves into an austenite matrix and also dissolves into a Ni₃Al-type gamma prime phase (γ′ phase), which is a precipitation strengthening phase, to thereby enhance the high-temperature strength of the alloy. Also, W precipitates a body-centered cubic α-(Mo, W) phase of a solid solution made of W and Mo into grain boundaries. W therefore functions to enhance the grain boundary strength of the alloy while also enhancing the machinability of the alloy. On the other hand, W acts to decrease the oxidation resistance. In view of enhancing the high-temperature strength and minimizing the decrease in oxidation resistance, the content of W in the Ni-based alloy in the present invention is 7.0 to 12.0%. To achieve the advantageous effects of W more reliably, the lower limit and the upper limit for W are preferably 10.0% and 11.0%, respectively.

<Mo: 4.0 to 11.0%>

Mo dissolves into an austenite matrix and also dissolves into a Ni₃Al-type gamma prime phase, which is a precipitation strengthening phase, to thereby enhance the high-temperature strength of the alloy. On the other hand, Mo acts to decrease the oxidation resistance. In view of enhancing the high-temperature strength and minimizing the decrease in oxidation resistance, the content of Mo in the Ni-based superalloy of the present invention is 4.0 to 11.0%. Note that it is preferable to set a preferable lower limit for Mo in consideration of the Ta content to be described later. To achieve the advantageous effect of Mo more reliably, in the case in which Ta is contained, the lower limit is preferably 4.5%. On the other hand, in the case in which Ta is not added, the lower limit for Mo is preferably 7.0% and more preferably 9.5%. Also, the upper limit for Mo is preferably 10.5 and more preferably 10.2%.

<Al: 5.0 to 7.5%>

Al binds to Ni to precipitate a Ni₃Al gamma prime phase. Thus, Al functions to enhance the high-temperature strength of the alloy and also produce an alumina film on the alloy surface and thereby impart oxidation resistance to the alloy. On the other hand, if the content of Al is excessively high, Al will act to excessively produce a eutectic gamma prime phase and thereby decrease the high-temperature strength of the alloy. In view of enhancing the oxidation resistance and the high-temperature strength, the content of Al in the Ni-based superalloy of the present invention is 5.0 to 7.5%. To achieve the advantageous effects of Al more reliably, its lower limit is preferably 5.5% and more preferably 6.1%, and the upper limit for Al is 6.7% and more preferably 6.5%.

<Cr: 0.5 to 7.5%>

Cr functions to promote formation of a continuous layer of alumina on the surface of the alloy or inside it and thereby improve the oxidation resistance of the alloy. To this end, it is necessary to add 0.5% or more of Cr. Also, in the case in which Cr is added along with Al, W, and Mo, high compressive proof strength is achieved at 1000° C. when Cr is 3.0 to 7.5%, as shown in table 4, FIG. 2, etc., to be discussed later. Further, high compressive proof strength can also be achieved at 1000 to 1100° C. when the content of Cr is 3.0% or lower. Note that adding 7.5% or more of Cr decreases the compressive proof strength at 1000° C. or higher and must therefore be avoided. By the present invention, it was discovered that adding Cr is not necessarily disadvantageous for the high-temperature strength but rather enhances the high-temperature strength and makes it possible to improve the oxidation resistance while maintaining high-temperature strength at a high degree on condition that 0.5 to 7.5% of Cr is added along with Al, W, and Mo. To achieve the advantageous effects of Cr more reliably, the lower limit and the upper limit for Cr are preferably 1.3% and 3.0%, respectively.

<Ta: 0.5 to 7.0%>

The Ni-based super heat-resistant alloy of the present invention may contain Ta. Ta dissolves into the Ni₃Al gamma prime phase in such a manner as to be replaced with the Al sites. Thus, Ta functions to enhance the high-temperature strength of the alloy and also improve the adhesion of the oxide film and enhance the oxidation resistance of the oxide film formed on the alloy surface and thereby improve the oxidation resistance of the alloy. On the other hand, if the content of Ta is excessively high, Ta will act to facilitate precipitation of harmful phases such as TCP (Topologically Close Packed) phases. In the case where Ta is contained in the present invention, the content of Ta is 0.5 to 7.0% in view of enhancing the oxidation resistance and the high-temperature strength and suppressing precipitation of harmful phases. To achieve the advantageous effects of Ta more reliably, the lower limit and the upper limit for Ta are preferably 2.5% and 6.5%, respectively.

<Other Optional Addable Elements>

The Ni-based super heat-resistant alloy of the present invention can contain one or more elements selected from Zr, Hf, rare-earth elements, Y, and Mg. Through segregation into the grain boundaries in the oxide film, Zr, Hf, the rare-earth elements, and Y suppress diffusion of metal ions and oxygen in the grain boundaries. This suppression of diffusion in the grain boundaries decreases the rate of growth of the oxide film, and also alters growth mechanisms that promote spallation of the oxide film and thereby improves the adhesion of the oxide film between the film and the alloy. In other words, these elements function to improve the oxidation resistance of the alloy via the decreasing of the growth rate and the improvement of the adhesion of the film mentioned above. Also, Mg forms a sulfide with S, which segregates into the interface between the oxide film the alloy and inhibits the chemical binding between them to decrease the tightness of contact of the film. Thus, Mg functions to prevent segregation of S to improve the adhesion and thereby improve the oxidation resistance of the alloy.

Note that, among the above rare-earth elements, La is preferably used since the oxidation-resistance improving effect of La is high. La also functions to prevent segregation of S in addition to the above-mentioned function to suppress diffusion, and these functions are good. For this reason, among the rare-earth elements, it is desirable to select La. Y also brings about the same advantageous effects as La. It is therefore preferable to add Y and particularly preferable to use two or more elements including La and Y.

In the case in which good mechanical characteristics are required in addition to oxidation resistance, it is preferable to use Hf or Zr and particularly preferable to use Hf. Moreover, in the case in which Hf is added, the oxidation resistance can be improved further by also adding Mg in addition to Hf since the S-segregation preventing effect of Hf is low. Thus, in the case in which certain mechanical characteristics are required as well as oxidation resistance, it is further preferable to use two or more elements including Hf and Mg.

If the amount of the above optional elements added is excessively large, the optional elements will excessively produce intermetallic compounds with, for example, Ni and decrease the toughness of the alloy. For this reason, it is preferable to contain these optional addable elements in respective preferable amounts.

In view of enhancing the oxidation resistance and suppressing decrease in toughness, the upper limit of the content of each of Zr and Hf in the present invention is 0.5%. The upper limit of the content of each of Zr and Hf is preferably 0.2% and more preferably 0.1%. The rare-earth elements and Y act to decrease the toughness to a greater extent than Zr and Hf do. The upper limit of the content of each of these elements in the present invention is 0.2%, preferably 0.1%, and more preferably 0.05%. In the case in which Zr, Hf, any of the rare-earth elements, and/or Y is contained, the lower limit is 0.001%. To sufficiently exhibit the advantageous effect of containing Zr, Hf, any of the rare-earth elements, and/or Y, the lower limit of the content is preferably 0.005% and more preferably 0.01% or more.

Also, as for Mg, it need be contained only in an amount necessary to form a sulfide with the impurity S contained in the alloy. Thus, the content of Mg is 0.001 to 0.03%. The upper limit for Mg is preferably 0.01%. On the other hand, to achieve the advantageous effect of adding Mg more reliably, the lower limit is preferably 0.005%.

The rest of the elements other than the above-mentioned addable elements are Ni and incidental impurities. In the Ni-based superalloy of the present invention, Ni is a main element that forms a gamma phase, and also forms a gamma prime phase with Al, Ta, Mo, and W. Note that among the above incidental impurity elements, S in particular is preferably 0.003% or less.

<Hot Forging Die>

In the present invention, at least one of the forming surface and the side surface of a hot forging die having the above alloy composition can be a surface having an antioxidant coating layer. In this way, it is possible to prevent oxidation of the die surface due to contact between the oxygen in air and the base material of the die at high temperature and scattering of scale resulting from it, and thus, more reliably prevent work environment deterioration and shape deterioration. The above antioxidant is preferably an inorganic material made of one or more of nitrides, oxides, and carbides in order to form a dense oxygen barrier film with a nitride, oxide, and/or carbide coating layer and prevent oxidation of the base material of the die. Note that the coating layer may be a single layer made of one of nitrides, oxides, and carbides or a laminate structure as a combination of two or more of nitrides, oxides, and carbides. Further, the coating layer may be a mixture made of two or more of nitrides, oxides, and carbides.

The above-described hot forging die using the Ni-based alloy for hot forging die of the present invention has high compressive strength at high temperature and good oxidation resistance and can prevent oxidation of the die surface due to contact between the oxygen in air and the base material of the die at high temperature and scattering of scale resulting from it, and thus more reliably prevent work environment deterioration and shape deterioration.

<Forged Product Manufacturing Method>

Representative steps in manufacturing a forged product with a hot forging die using the Ni-based alloy for hot forging die of the present invention will be described.

First of all, in the first step, a forging workpiece is heated to a predetermined forging temperature. Since the forging temperature varies from one material to another, the temperature is adjusted as appropriate. The hot forging die using the Ni-based alloy for hot forging die of the present invention has such characteristics that it is capable of isothermal forging or hot die forging even under a high-temperature environment in air. Thus, the hot forging die is advantageous for hot forging of materials such as Ni-based super heat-resistant alloys and Ti alloys, which are known as poor workability materials. Representative forging temperatures are within the range of 1000 to 1150° C.

Then, the forging workpiece heated in the first step is hot-forged using the hot forging die (second step). In the case of the above hot die forging or isothermal forging, the hot forging in the second step is preferably closed die forging. Also, as mentioned above, the Ni-based alloy for hot forging die of the present invention enables hot forging at a high temperature of 1000° C. or higher in air in particular by adjusting the content of its component Cr.

EXAMPLES

The present invention will be described more specifically by way of the following examples. Ingots of Ni-based alloys for hot forging die shown in Table 1 were manufactured by vacuum melting. Its units are given in mass %. Note that the content of each of P, S, N, and O contained in the below ingots is 0.003% or less, and the content of each of C, Si, Mn, Co, Ti, Nb, and Fe is 0.03% or less.

TABLE 1 (mass %) Mo W Al Cr Ta Hf Zr La Y Mg Balance Example 1 10.0 10.6 6.2 1.5 — — 0.08 — 0.03 — Ni and Example 2 10.0 10.8 6.2 3.7 — — 0.05 — 0.03 — inevitable Example 3 9.8 10.6 6.2 7.1 — — 0.03 — 0.04 — impurities Example 4 10.0 10.6 6.2 1.5 — — — — — — Example 5 10.0 10.6 6.2 1.5 3.1 — — — — — Example 6 8.0 10.2 6.1 1.5 2.8 0.15 — — — — Example 7 7.9 10.2 6.1 1.5 2.8 — — 0.08 — — Example 8 7.9 10.2 6.1 1.5 2.6 0.15 — 0.07 — — Example 9 4.9 11.0 5.5 1.6 6.3 — — — — 0.017 Example 10 4.9 11.0 5.5 1.6 6.4 0.17 — — — 0.017 Comparative 10.4 10.7 6.3 — — — — — — — example 1 Comparative 10.0 10.6 6.2 — — — 0.08 — 0.03 — example 2 * “—” means the element was not added.

A 10-mm cube was cut out from each of the above ingots, and the surface was polished close to #1000 finish to thereby prepare an oxidation-resistant test piece, and the oxidation resistance was evaluated. In the oxidation resistance test, two types of tests were performed: a test simulating long-duration use and a test simulating repetitive use in air as a die for hot forging.

As the oxidation resistance test simulating long-duration use, each of the test pieces in examples 1 to 10 and comparative examples 1 and 2 was used to perform a heating test in which the test piece was placed in a ceramic crucible made of SiO₂ and Al₂O₃, introduced into a furnace heated to 1100° C., and held therein at 1100° C. for a predetermined time, the crucible with the test piece placed therein was taken out of the furnace, and immediately after the crucible was taken out, a lid of the same material was put on the crucible in order to prevent separation of scale to outside of the crucible, and the crucible was air-cooled. In the heating test, each test piece was subjected to a test in which the holding duration was three hours and a test in which the holding duration was eight hours, in order to evaluate the oxidation resistance against long-duration use.

For each test piece, before each heating test, the surface area of the test piece and the mass of the crucible with the test piece placed therein were measured. After the heating test, the test piece was cooled to room temperature, and the mass of the crucible with the test piece placed therein was measured. The mass measured before each test was subtracted from the mass measured after the test, and the resultant value was divided by the surface area measured before the test to thereby calculate the mass change of the test piece after the test per unit surface area. The larger the value of the mass change, the larger the amount of scale generated per unit area. The mass change was calculated as below.

Mass Change=(Mass after Test−Mass before Test)/Surface Area before Test

Table 2 shows the mass change of each test piece per unit surface area calculated after each of the heating tests with the above holding durations. The unit of the mass change is mg/cm².

From Table 2, it can be observed that, in examples 1 to 10 of the present invention, in which Cr was added, the amount of scale generated was reduced and the mass change after eight hours was less than half of that in comparative examples 1 and 2, in which Cr was not added, and that good oxidation resistance against long-duration use was achieved owing to adding Cr.

TABLE 2 Mass change Mass change after 3 hours after 8 hours Example 1 0.3 0.7 Example 2 0.1 0.3 Example 3 <0.1 <0.1 Example 4 <0.1 0.3 Example 5 <0.1 0.1 Example 6 0.2 0.1 Example 7 1.1 1.1 Example 8 1.7 1.8 Example 9 0.3 0.2 Example 10 0.3 0.3 Comparative example 1 1.6 4.6 Comparative example 2 3.1 5.0

As the oxidation resistance test simulating repetitive use, each of the test pieces in examples 1 and 4 to 10 and comparative example 1 was used to perform a heating test in which the test piece was placed on a ceramic container made of SiO₂ and Al₂O₃, introduced into a furnace heated to 1100° C., held therein at 1100° C. for three hours, taken out of the furnace, and air-cooled. The heating test was repeated five times by introducing the test piece again after it was cooled, in order to evaluate the oxidation resistance against repetitive use.

For each test piece, before the first heating test, the surface area and mass of the test piece were measured. After each of the first to fifth heating tests, the mass of the test piece was measured after it was cooled to room temperature and the scale on its surface was removed with a blower. The mass measured before the first test was subtracted from the mass measured after each test, and the resultant value was divided by the surface area measured before the first test to thereby calculate the change in mass of the test piece after each test per unit surface area. The larger the absolute value of the mass change, the larger the amount of scale scattered per unit area. The mass change after each repetition was calculated as below.

Mass change=(Mass after test−Mass before first test)/Surface area before first test

Table 3 shows the mass change of each test piece per unit surface area calculated after each heating test. The unit of the mass change is mg/cm². Also, FIG. 1 shows the correlation between the number of times a heating test was performed and the change in mass.

As shown in Table 3 and FIG. 1, it can be observed that with the alloys in examples 1 and 4 to 10 of the present invention, the generation (scattering) of scale was reduced and the absolute value of the mass change was small and the oxidation resistance against repetitive use was therefore good compared to the alloy in comparative example 1. It can be observed that, among them, in particular in example 1, in which Y and Zr were added in addition to Cr, and example 5, in which Ta was added in addition to Cr, the scattering of scale was reduced and the oxidation resistance against repetitive use was particularly good as compared to example 4, in which only Cr was added. Also, it can be observed that in examples 6 to 10, in which Hf, La, or Mg was added in addition to Cr and Ta, the oxidation resistance against repetitive use was even better than examples 1 and 5.

TABLE 3 Mass Mass Mass Mass Mass change change change change change after after after after after first test second test third test fourth test fifth test Example 1 −0.8 −2.0 −2.7 −3.0 −3.5 Example 4 0.0 −2.7 −9.2 −19.5 −27.6 Example 5 −0.1 −1.1 −1.7 −3.7 −4.7 Example 6 0.2 0.2 −0.5 −0.9 −1.0 Example 7 0.5 0.9 1.0 1.0 1.2 Example 8 0.8 1.1 0.9 0.9 0.9 Example 9 0.2 0.5 −0.1 −0.2 −0.2 Example 10 0.3 0.3 0.2 0.2 0.2 Comparative −2.3 −8.6 −18.0 −29.5 −39.0 example 1

Next, a workpiece to be collected as a test piece measuring 8 mm in diameter and 12 mm in height was cut out from each of the ingots in examples 1 to 10 and comparative examples 1 and 2 in Table 1, and the surface was polished close to #1000 finish to thereby prepare a compression test piece. Using this compression test piece, a compression test was performed at temperatures of 900° C., 1000° C., and 1100° C. under conditions of a strain rate of 10⁻³/sec and a compression ratio of 10%. The compressive strength at high temperature was evaluated by deriving the 0.2% compressive proof strength from a stress-strain curve obtained by the compression test. This compression test is intended to test whether the test piece has sufficient compressive strength at high temperature as a die for hot forging. If the compressive proof strength is at least 300 MPa, the test piece can be considered to have sufficient strength, and the compressive proof strength is preferably at least 350 MPa.

Table 4 shows the 0.2% compressive proof strength of each of the test pieces in examples 1 to 10 and comparative examples 1 and 2 at each test temperature. Also, FIG. 2 illustrates the correlation between each test temperature and the 0.2% compressive proof strength in examples 1 to 5 and comparative example 1.

From Table 4, it can be observed that the compressive proof strength at 1000° C. and a strain rate of 10⁻³/sec was 500 MPa or higher in examples 1 and 5 and that the compressive proof strength at 1100° C. and a strain rate of 10⁻³/sec was 350 MPa or higher in examples 1 and 4 to 10, in each of which Cr was given in a preferable amount. Also, from FIG. 2, it is obvious that the compressive proof strength at 1000° C. in examples 1 to 5, in which the amount of Cr added was at or below the upper limit, or 7.5%, was close to or higher than that in comparative example 1, in which no Cr was contained, and that the compressive proof strengths at 1000 to 1100° C. in examples 4 and 5, in which the amount of Cr added was 3.0% or less, were close to or higher than those in comparative example 1. The above indicates that any of the alloys of the present invention had high compressive strength at high temperature.

TABLE 4 900° C. 1000° C. 1100° C. (MPa) (MPa) (MPa) Example 1 740 614 378 Example 2 775 577 310 Example 3 686 503 256 Example 4 — 684 376 Example 5 — 638 489 Example 6 — — 440 Example 7 — — 380 Example 8 — — 382 Example 9 — — 396 Example 10 — — 400 Comparative Example 1 — 504 390 Comparative Example 2 752 648 303 * “—” means the test was not performed.

Next, a die satisfying the composition of the hot die Ni-based alloy of the present invention as shown in Table 5 and having its surface coated with an antioxidant made of oxides shown in Table 6 was used to evaluate the antioxidant's effect of preventing oxidation of the die for hot forging die and preventing scattering of scale.

FIG. 3 shows an image of the exterior of the die for the hot forging die coated with the antioxidant captured after the die for the hot forging die was heated to 1000° C. or higher in air. As is observed from FIG. 3, the antioxidant applied to the surface of the die for hot forging die did not peel. In addition, no scattering of scale was observed. These indicate that the antioxidant prevents oxidation of the die and scattering of scale.

TABLE 5 (mass %) Mo W Al Cr Balance 9.9 10.7 6.2 1.5 Ni and inevitable impurities* *Incidental impurities (P, S: <0.003%, C, Si, Mn, Co, Ti, Nb, Fe: <0.03%)

TABLE 6 (mass %) SiO₂ B₂O₃ Al₂O₃ CaO Balance 53.0 5.6 12.7 18.0 Trace oxides, etc. added* *Trace oxides added (Na₂O: 0.6%, K₂O: 0.1%, Fe₂O₃: 0.2%, MgO: 0.5%, TiO₂: 0.6%, SrO: 0.2%)

From the above results, it is observed that the Ni-based alloy for hot forging die of the present invention exhibits both sufficient oxidation resistance and high compressive strength at high temperature even when used in hot forging in air. In particular, the hot die Ni-based alloy can significantly reduce spallation of scale and therefore suppress work environment deterioration and shape deterioration.

In particular, by preparing a hot forging die from the Ni-based alloy for hot forging die of the present invention and forming an antioxidant coating layer on at least one of its forming surface and side surface, it is possible to prevent work environment deterioration and also shape deterioration to a greater extent. This indicates that a hot forging die made from the Ni-based alloy for hot forging die of the present invention is advantageous for hot die forging and isothermal forging in air. 

1. A Ni-based alloy for hot forging die, comprising, by mass, 7.0 to 12.0% W, 4.0 to 11.0% Mo, 5.0 to 7.5% Al, 0.5 to 7.5% Cr, and the balance of Ni with inevitable impurities.
 2. The Ni-based alloy for hot forging die according to claim 1, further comprising 0.5 to 7.0% by mass Ta.
 3. The Ni-based alloy for hot forging die according to claim 1, further comprising one or more elements selected from, by mass, 0.001 to 0.5% Zr, 0.001 to 0.5% Hf, 0.001 to 0.2% a rare-earth element, 0.001 to 0.2% Y, and 0.001 to 0.03% Mg.
 4. The Ni-based alloy for hot forging die according to claim 1, wherein a 0.2% compressive proof strength of the Ni-based alloy for hot forging die at a test temperature of 1000° C. and a strain rate of 10⁻³/sec is at least 500 MPa.
 5. The Ni-based alloy for hot forging die according to claim 1, wherein a 0.2% compressive proof strength of the Ni-based alloy for hot forging die at a test temperature of 1100° C. and a strain rate of 10⁻³/sec is at least 350 MPa.
 6. A hot forging die using the Ni-based alloy for hot forging die according to claim
 1. 7. The hot forging die according to claim 6, further comprising an antioxidant coating layer on at least one of a forming surface and a side surface of the hot forging die.
 8. A method for manufacturing a forged product, comprising: a first step of heating a forging workpiece; and a second step of hot-forging the forging workpiece heated in the first step by using the hot forging die according to claim
 6. 9. The method for manufacturing a forged product according to claim 8, wherein the second step is performed with the hot forging die heated to at least 1000° C. 